1. Field of the Invention
The present invention relates to a cooling or refrigerating system having at least two compressors of different capacities and one evaporator, and more particularly, to such a system that regulates the air flow through the evaporator.
2. Description of the Related Art.
In a conventional cooling or refrigeration system, a compressor pumps a refrigerant gas under pressure to a condenser unit where heat is dissipated causing the refrigerant to liquify. After being passed through a thermostatic expansion valve or equivalent means, the refrigerant boils thereby absorbing heat (cooling) from the evaporator coils. Typically, a fan is positioned in front of the evaporator and cool air is blown to the area being cooled or refrigerated. As discussed in Modern Refrigeration and Air Conditioning, Althouse, A.D. et al, 1979 edition, pp. 150-151, the refrigerant enters the evaporator coils in liquid state and starts vaporizing as it progresses towards the low side of the evaporator. The best superheat setting for an evaporator is the point at which the temperature of the thermal bulb of the thermostatic expansion valve changes the least when the system is running. This setting is called the Minimum Stable Signal (MSS) point or setting. This setting is a result of the evaporator flow, the behavior of the expansion valve and the displacement of air passed through the cooling evaporator coils. The expansion valve is usually calibrated during the installation of the system. The flow characteristics and the air blown are usually constant. Therefore, for a one compressor system, an initial MSS setting is usually satisfactory until the components are worn out and the characteristics of the system change. However, when two compressors of different capacities are used, there may be considerable savings of energy by having a lower rating compressor `cut in`, as discussed in U.S. Pat. No. 4,535,602 issued to Alsenz in Aug. 20, 1985. But the superheat also moves away considerably from the MSS setting in the device disclosed by Alsenz. This changes the efficiency of the system. To solve this problem the present invention also regulates the amount of air blown through the evaporator coils. If less air is blown through the evaporator coils then less heat is absorbed and the lower capacity of the `cut in` compressor is compensated urging the superheat setting back to the MSS setting. When the larger compressor `cut in` then the speed of the evaporator fan is increased again.
Other patents describing the closest subject matter provide for a number of more or less complicated features that fail to solve the problem in an efficient and economical way. None of these patents suggest the novel features of the present invention.